1. Field of the Invention
The present invention relates to a dust boot made of thermoplastic synthetic resin by means of blow molding, the dust boot having ring portions at opposite ends thereof by means of which it is attached in protective mounting on a component to be protected such as an automobile ball joint. The portion of the boot between the attachment ring portions is formed into the shape of an accordion or bell. The invention also relates to a method for making such a dust boot.
The dust boots referred to herein include dust boots for ball joints used in the automobile industry and the machine industry for steering rack boots, constant velocity joint boots and soon.
2. Description of the Prior Art
Conventional dust boots and methods of making them are explained hereafter, taking as an example, the rack boot shown in FIG. 10, which boot is made in an elastic accordion shape and is used for protectively covering an automobile rack and pinion steering mechanism.
Many dust boots of the above kind are made of rubber, e.g., a rubber of the chloroprene group. However, in order to improve product moldability, heat resistance, cold resistance, shock resistance against impact with gravel, etc., these boots are now being made from a thermoplastic resin. Such can be a thermoplastic elastomer of any one or more of the polyolefin, polyurethane, polyester, polyvinyl chloride, and polyamide groups. (Resins such as those mentioned above are hereinafter generally referred to simply as "resin".) Conventional methods of molding these resin dust boots, include an extrusion blow molding method, which involves wrapping an extruded cylindrical resin parison with a metal cavity mold for blow molding. The parison is then expanded by blowing compressed air therein to press the parison against the inner surface of the cavity and to thus obtain a molded piece also, an injection blow molding method can be used, which involves molding of a parison by injecting resin through an injection molding cylinder to the space between a cavity metal mold and a core metal mold, moving the parison together with the core metal mold to a blow molding cavity mold and closing the mold around the parison. The parison is then expanded by blowing compressed air into it for pressing it against the inner surface of the cavity.
As shown in FIG. 10, the size of the attachment ring portions at both ends of dust boot 1, especially the inner circumference of attachment ring portion 2 with the smaller diameter, is, in a typical case, made slightly smaller than the diameter of the locking groove part 4 of receiving shaft 3. This is so that there can be an interference fit between the attachment ring and receiving shaft thereby to securely attach ring portion 2 in locking groove 4. This secure attachment is intended to provide sealing effective to prevent dust and muddy water from entering the boot. In order to further improve sealing ability, it is general practice to form a protruding lip on the inner surface of attachment ring portion 2. The attachment ring portion 2 is formed, in case of the extrusion blow molding method, by means of compression molding, which involves sandwiching the parison between the core metal mold and the cavity metal mold, thereby limiting the inner diameter thereof, and thus forming attachment ring portion 2. On the other hand, in case of injection blow molding, attachment ring portion 2 is molded by injection molding between the cavity metal mold and the core metal mold when forming the parison. At the time of blow molding, the accordion shaped portion 5 molded, while limiting the inner diameter of the attachment ring portion 2, which is positioned between the cavity metal mold and the core metal mold without any space in between.
The conventional method of molding attachment ring portion 2 presents a problem in that, when accordion portion 5 is formed by blow molding the parison in the cavity metal mold. The part of attachment ring portion 2 which connects to accordion shaped portion 5, is stretched towards accordion portion 5, allowing compressed air, at a pressure typically between 4 and 7 kg/cm.sup.2, to flow past the inner side of attachment ring portion 2. This impairs the measurement accuracy of the inner surface of attachment ring portion 2 and makes it difficult to so form vertical wall portion 7, which bends outwardly radially from inner end portion 6 of attachment ring portion 2 and substantially perpendicular to the attachment ring axis. In other words, with conventional practice it is difficult to form the inner end portion 6 into a sharp-angled shape. Unless inner end portion 6 is of a sharp-angled shape, locking of the inner end portion 6 with locking surface 4a at one end of locking groove 4 becomes weak when attaching the attachment ring portion 2 to the locking groove 4. This causes the inner end portion to easily come out of the locking groove. Moreover, if the measurement, i.e., diameter of the inner surface of attachment ring portion 2 is not accurate, in addition to making it difficult to obtain good sealing ability, this diameter error permits play between the width of attachment ring portion 2 and the width of locking groove 4. This in turn allows for slippage in the direction of receiving shaft 3 at the time of extension and contraction of the dust boot 1. Wear of the inner surface of attachment ring portion 2 and further impairment the sealing ability results.